Fachbereich Physik
Universität Konstanz
Refractive Index Modification by Synchrotron Radiation
Diploma thesis
at the
Centre for Atom Optics and Ultrafast Spectroscopy (CAOUS) Swinburne University of Technology, Melbourne
submitted by
Felix Salomon
Supervisor: Dr. Paul Stoddart, Swinburne University
1. Assessor: Prof. Dr. Alfred Leitenstorfer, Universit¨at Konstanz 2. Assessor: Prof. Dr. Paul Leiderer, Universit¨at Konstanz
Date of submission: 26. June 2007 Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2007/4425/
URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-44255
ii
Parts of this diploma thesis were presented at the
1
stInternational Workshop on Multiphoton Processes in Glass and Glassy Materials
”Special Session: Optical Material Processing by Synchrotron Radiation”
Darlington Centre, University of Sydney, Sydney, Australia Dec. 11-12 2006
I, Felix Salomon, declare that this thesis entitled
”Refractive Index Modification by Synchrotron Radiation“
is my own work and has not been submitted previously in whole or in part, in respect of any other academic award.
Hiermit erkl¨are ich,Felix Salomon, geboren am 28.02.1980 in Konstanz, 1. dass ich meine Diplomarbeit mit dem Titel:
”Refractive Index Modification by Synchrotron Radiation“
an der Swinburne University in Melbourne, Australien unter Anleitung von Dr. Paul Stoddart selbst¨andig und ohne fremde Hilfe angefertigt habe und keine anderen als die angef¨uhrten Hilfen benutzt habe;
2. dass ich die ¨Ubernahme w¨ortlicher Zitate, von Tabellen, Zeichnungen, Bildern und Programmen aus der Literatur oder anderen Quellen (Internet) sowie die Verwendung der Gedanken anderer Autoren an den entsprechenden Stellen innerhalb der Arbeit gekennzeichnet habe.
Ich bin mir bewusst, dass eine falsche Erkl¨arung rechtliche Folgen haben wird.
. . . . Konstanz, 26.06.2007
Contents
1 Abstract/Kurzfassung 1
2 Introduction 5
2.1 Applications . . . 5
2.1.1 Fibre Bragg gratings . . . 5
2.1.2 Long period gratings . . . 7
2.1.3 Planar waveguides . . . 8
2.2 Motivation for synchrotron radiation . . . 9
2.3 Previous research on synchrotron light . . . 9
2.3.1 Synchrotron radiation applied to grating and waveguide fab- rication . . . 10
3 Theory 13 3.1 Photosensitivity . . . 13
3.1.1 Enhancing the photosensitivity in germanosilica . . . 13
3.1.2 Models for photosensitivity . . . 15
3.2 Synchrotron radiation . . . 20
3.3 Ellipsometry . . . 21
3.3.1 Polarisation . . . 22
3.3.2 Phase-modulated ellipsometry . . . 23
3.3.3 Performing ellipsometry measurements . . . 24
4 The synchrotron experiments 27 4.1 The National Synchrotron Research Center . . . 27
vi Contents
4.1.1 The beamline . . . 29
4.1.2 The experimental unit . . . 29
4.2 Sample materials . . . 31
4.3 Sample preprocessing . . . 31
4.4 The X-ray mask . . . 33
4.5 Sample irradiation . . . 34
4.5.1 Standard glasses . . . 35
4.5.2 Special glasses . . . 35
4.5.3 Optical fibres . . . 36
4.6 Qualitative results . . . 36
5 Measurements 39 5.1 Surface analysis . . . 39
5.1.1 Atomic force microscope . . . 39
5.1.2 Stylus profilometry . . . 40
5.2 Refractive index . . . 41
5.2.1 Modeling the refractive index . . . 41
5.2.2 Ellipsometry . . . 44
5.2.3 Analysis of fibres . . . 44
6 Analysis and results 45 6.1 Surface analysis . . . 45
6.1.1 Atomic force microscope . . . 45
6.1.2 Stylus profilometry . . . 47
6.1.3 Problems performing surface analysis . . . 47
6.1.4 Summary of the surface analysis results . . . 48
6.2 Refractive index . . . 49
6.2.1 Modeling . . . 50
6.2.2 Ellipsometry . . . 52
7 Conclusion 55
A Differential Interference Contrast measurements 57
Acknowledgments 61
Bibliography 63
List of Figures
2.1 Schematic of a fibre Bragg grating . . . 6
2.2 Schematic of a long period grating . . . 7
2.3 Schematic of a fibre grating writing setup . . . 8
2.4 Schematic of a planar waveguide . . . 8
3.1 Effect of hydrogenation on photo-induced index changes . . . 14
3.2 Effect of flame brushing on photo-induced index changes . . . 16
3.3 Transmission loss spectrum of a fibre preform . . . 17
3.4 Dependency of the refractive index on the density in silica . . . 19
3.5 Electron emitting synchrotron radiation . . . 20
3.6 Brightness and radiation spectrum of the NSRRC . . . 21
3.7 Ellipsometry principles . . . 22
3.8 Schematic of a phase-modulated ellipsometer . . . 23
3.9 Ellipsometry flowchart . . . 24
4.1 Layout of the NSRRC . . . 27
4.2 Schematic of the LIGA beamline . . . 29
4.3 Radiation spectrum of the LIGA beamline . . . 30
4.4 Deposited dose in silica glass . . . 31
4.5 Mask specifications . . . 33
4.6 Mounting of the samples . . . 35
4.7 Mask layout and duration of exposure . . . 36
4.8 Exposure regions of chalcogenide glass . . . 36
4.9 Samples after irradiation . . . 37
viii List of Figures
5.1 Position of the scan spots . . . 40
5.2 Densification of the exposed material . . . 41
5.3 The layer model of compaction . . . 42
6.1 AFM surface scan of borosilicate glass . . . 45
6.2 AFM height profile of borosilicate glass . . . 46
6.3 Stylus hight profile of SF-10 glass . . . 47
6.4 Stylus hight profile of fused silica . . . 48
6.5 Compaction of the samples . . . 49
6.6 Calculated index profile in borosilicate (simple model) . . . 50
6.7 Calculated index profile in borosilicate (layer model) . . . 51
6.8 Ellipsometry results SF-10 . . . 52
6.9 Beam cross section and backside reflection . . . 53
A.1 Schematic of the LPG mask . . . 58
A.2 Experimental DIC setup . . . 58
A.3 DIC images of GF1 fibre . . . 59
A.4 Line scan across DIC image . . . 59
A.5 Vertical line scans along DIC image . . . 60
List of Tables
4.1 List of exposed standard glasses . . . 32
4.2 List of exposed special glasses . . . 32
4.3 List of exposed optical fibres . . . 32
6.1 Summary of the step heights . . . 49
6.2 Summary of the calculated relative refractive index change . . . 51
Chapter 1
Abstract/Kurzfassung
Refractive index modifications in glass play an important role in fibre optic com- munications. One of the more challenging issues in fibre optics has been how to process the light in optical systems while avoiding the losses which are inherent in the process of coupling light out of fibres. Using bulk optics to perform reflection, diffraction and filtering, inevitably increases the system complexity and at the same time decreases its performance. Hence there is great interest in developing in-fibre equivalents of devices such as beam splitters, filters or optical mirrors as they would potentially perform this processing more efficiently. Moreover, ’fiberised’ devices would increase the stability, reliability and simplicity of fibre optic communications.
To date, the common method used to write refractive index structures into glass involves the use of a UV laser source. This method is constrained by a number of restrictions which could possibly be avoided by using the high energy X-ray light of a synchrotron to produce in-fibre devices. Mid last century, researchers performed experiments on the exposure of glass to synchrotron light. This diploma thesis expands on these initial results with the benefit of modern experimental facilities, to investigate the effects of synchrotron radiation on different glass samples and intends to thereby gain a more coherent, fundamental understanding of the effects of synchrotron light.
In order to produce appropriate samples, various optical glasses and fibres were taken to theNational Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Tai- wan, and irradiated with synchrotron light with an energy above 500 eV and a peak energy of 7.8 keV. Tests were subsequently performed to measure the compaction (densification) of the material and possible changes in the refractive index. By means of surface analysis, evidence has been found that compaction occurred and this den-
2 1. Abstract/Kurzfassung sification has been quantified. A model for the refractive index modification has been developed and, using ellipsometry, initial measurements have been performed to prove the model.
Based upon the work that has been done in the course of this diploma thesis, more specific research in the form a PhD project is to be performed so as to produce optical devices using this technology.
Die Modifizierung des Brechungsindex von Glas ist von herausragender Bedeutung in der Faseroptik. Dabei stellt die Vermeidung von Verlusten bei der Auskopplung von Licht aus Glasfasern eine besondere Herausforderung dar. Durch die Verwendung von zus¨atzlichen optischen Bauelementen zur Realisierung von Reflexion, Brechung und Filterung des Lichts ist es unvermeidlich, daß sich die Komplexit¨at des Systems erh¨oht und sich gleichzeitig die Effizienz verringert. Daher besteht großes Interesse an der Entwicklung von “in-fibre” Bausteinen wie zum Beispiel Strahlteiler, Filter oder optische Spiegel, da diese eine h¨ohere Effizienz bei der gleichen Anwendung versprechen. Dar¨uber hinaus k¨onnten solche integrierten Elemente die Stabilit¨at, Zuverl¨assigkeit und Einfachheit der Faseroptiken in der Telekommunikation erh¨ohen.
Heutzutage werden Strukturen mit variierendem Brechungsindex in Glas mit Hilfe eines UV Lasers erzeugt. Diese Methode unterliegt einer Reihe von Einschr¨ankun- gen. Durch die Verwendung hochenergetischer Synchrotronstrahlung zur Herstel- lung von in-fibre Elementen, besteht die M¨oglichkeit, diese Nachteile zu vermei- den. Bereits in den 50er Jahen wurden Experimente durchgef¨uhrt, die den erfol- greichen Nachweis von Strukturver¨anderungen in Glas durch die Einwirkung von Synchrotronstrahlung erbrachten. Die vorliegende Diplomarbeit m¨ochte auf diesen Erkenntnissen mit Hilfe moderner, damals noch nicht vorhandener Meßtechniken aufbauen mit dem Ziel, ein zusammenh¨angenderes, grundlegendes Verst¨andnis zu vermitteln, welches den Effekt von Synchrotronstrahlung auf Glas beschreibt.
Hierzu wurden verschiedene optische Gl¨aser und Glasfasern amNational Synchrotron Radiation Research Center(NSRRC) in Hsinchu, Taiwan, mit Synchrotronstrahlung bestrahlt. Die Energie der Photonen lag dabei oberhalb 500 eV, das Maximum des Spektrums lag bei etwa 7.8 keV. Im Anschluss daran wurden die Proben im Hinblick auf eine Kompaktion (Verdichtung) des Materials und eine m¨ogliche ¨Anderung des Brechungsindex untersucht. Mit Hilfe von AFM-Analysen und Stylus Profilometrie konnte gezeigt werden, daß eine Verdichtung auf Grund der Strahlungseinwirkung stattgefunden hat. Der Grad der Dichte¨anderung wurde gemessen. Ein Modell wurde entwickelt, um die ¨Anderung des Brechungsindex zu simulieren und mit Hilfe
3 von Ellipsometrie wurden erste Messungen durchgef¨uhrt, um das Modell zu veri- fizieren.
Die Erkenntnisse, die im Verlauf dieser Diplomarbeit gewonnen wurden, bilden die Grundlage f¨ur weiterf¨uhrende Untersuchungen im Hinblick auf die Produktion von integrierten optischen Bauelementen mit Hilfe von Synchrotronstrahlung.
Chapter 2
Introduction
Modification of the refractive index of glass is a basic principle underpinning vari- ous applications in modern fibre optics and telecommunication. This chapter will introduce some of the most important devices whose functionality is based on a mod- ulation of the refractive index. An overview will be given of existing research which is relevant to refractive index changes in glass. In particular research is considered which has been done on the effect of synchrotron radiation on silica glass.
2.1 Applications
One group of in-fibre devices are fibre gratings which display a periodic modulation of the refractive index in the fibre core. There are two different types of fibre gratings, each with their specific suitability: fibre Bragg gratings and long period gratings.
2.1.1 Fibre Bragg gratings
The most basic type of fibre grating is known as a fibre Bragg grating (FBG). This consists in a periodic pattern with an increased refractive index in the core of a optical single-mode fibre. This uniform type of fibre grating is considered to be the fundamental building block for most Bragg grating structures. Typically, the index divergence is approximately periodic over a length of a few millimetres, the period is of the order of hundreds of nanometres. A narrow range of wavelengths of the light propagating along the fibre core satisfy a Bragg condition given by
λB = 2nef fΛ (2.1)
where λB is the Bragg wavelength or centre wavelength of the grating, nef f is the effective refractive index of the fibre and Λ is the period of the refractive index mod- ulation. Wavelengths which match the Bragg condition are reflected at the different
6 2. Introduction parts of the grating in phase and add up constructively, while other wavelengths are nearly not affected by the Bragg grating, except for some side lobes which frequently occur in the reflection spectrum. The reflection of light that is not coincident with the Bragg wavelength resonance is very weak at each of the grating planes because of the index mismatch. For example, a 1 mm grating at a wavelength of 1.5 µm with a ∆n of 10−3 will reflect about 0.05 % of the off-resonance light. In order to suppress these sidebands the grating can be designed in an aperiodic way so that the index contrast is reduced towards the ends of the grating.
1549,60 1549,8 1550,0 1550,2 1550,4 20
40 60 80 100
core cladding
periodic modulation of the refractive index
(a) (b)
Reflectivity (%)
Wavelength (nm)
Figure 2.1: (a) Schematic of a fibre Bragg grating. (b) Typical reflectivity spec- trum of a FBG.
The refractive index profile of a uniform Bragg grating formed within the core of an optical fibre with average indexn0 can be expressed as
n(z) =n0+ ∆ncos
µ2πz Λ
¶
(2.2) where ∆n is the amplitude of the induced refractive index change (typical values of ∆n are 10−5 to 10−3), and z is the longitudinal distance along the fibre. From the coupled mode theory of Lam and Garside [30] to describe the reflectivityR(l, λ) of fibre gratings as a function of grating length l and wavelength λ, we obtain the following expression:
R(l, λ) = Ω2sinh2(sl)
∆k2sinh2(sl) +s2cosh2(sl). (2.3) Here, Ω is the coupling coefficient which is proportional ∆n/λ; ∆k = k − π/λ is called the detuning wavefactor, k = 2πn0/λ is the propagation constant, and s2 = Ω2 −∆k2. At the Bragg grating centre wavelength there is no wavefactor detuning and ∆k = 0; therefore the reflectivity becomes
R(l, λ) = tanh2(Ωl). (2.4)
The reflectivity increases as the induced index change increases. Similarly, as the length of the grating increases so does the resultant reflectivity. Due to the strong de- pendency of the Bragg grating dimensions on environmental factors such as strain,
2.1. Applications 7 temperature and bend radius, changes in these conditions give rise to a measur- able change of the reflectivity and thereby make for various applications in sensor technique. In telecommunications FBG’s are used for wavelength filtering, e.g. for combining or separating multiple wavelength channels in wavelength division mul- tiplexing systems. In laser and sensor applications, fibre Bragg gratings can be designed to act as narrow-band high-reflectance mirrors.
2.1.2 Long period gratings
In contrast to a FBG, a long period grating (LPG) comprises a periodic modulation of the refractive index over the length of several centimetres with a period L of 100µm to 1 mm. Unlike in the FBG the wavelengths satisfying the phase matching condition
(ncoreef f −ncladef f)L=λ (2.5) are not reflected but are instead coupled out into the cladding of the fibre propagating in the same direction. The excited cladding mode attenuates in the coated fibre part after the grating, which results in the appearance of selective resonance losses in the transmission spectrum. In contrast to fibre Bragg gratings, a LPG does not produce reflected light and can serve as spectrally selective absorber.
800 850 900 950 1000 1050 0
10 20 30 40 50 60 70 80 90
core cladding 100
periodic modulation of the refractive index
(a) (b)
centre wavelength
cladding mode
Transmission (%)
Wavelength (nm)
Figure 2.2: (a) Schematic of a long period grating in an optical fibre and the cou- pling of the centre wavelength into the cladding. (b) Typical transmission spectrum of a LPG.
The centre wavelengths of the attenuation bands are dependent upon the composi- tion of the fibre and are again influenced by strain, temperature, bend radius and additionally the refractive index of the material surrounding the fibre. This leads to various applications in sensor technique. In fibre amplifiers a LPG can be used to remove unwanted frequencies.
Writing either of the long period and fibre Bragg gratings into the fibre core requires a very sensitive mask setup an example of which is shown in Figure 2.3 on the following page.
The mask generates a pattern of constructively superpositioned light at the position of the fibre core and the resulting regions of different intensities create the desired
8 2. Introduction
Ultraviolet light Phase mask
Fibre
Interference pattern in fibre core
Figure 2.3: Schematic of a common FBG writing set-up.
index structure in the fibre. During irradiation the fibres need to be aligned very accurately and only one fibre at a time can be processed. Because of the strong absorption of the light in the material of the coating, the fibre has to be stripped prior to the exposure. Moreover the core material is not initially very responsive to the UV light but has to be pre-treated in order to produce the required index changes.
2.1.3 Planar waveguides
Planar waveguides are waveguides with a planar geometry guiding light only in one dimension. The principle underpinning planar waveguides is that a propagating beam of light is guided by total reflection in an area with a higher refractive index than the surrounding material.
Substrate Guiding channel
Figure 2.4: Schematic of a planar waveguide. The blue line represents the actual waveguide with a refractive index n1 greater than the index n0 of the surrounding material.
Single mode planar waveguides are commonly manufactured in materials with fem- tosecond lasers. At 775 nm, glass is transparent to incident light. Ultrafast laser pulses melt the glass locally via confined multi-photon absorption and avalanche ionisation inside the bulk material. The glass then resolidifies, changing its physical properties. The result is an index gradient that acts like a waveguide. A beam of light propagating along the same path in the glass will be guided in the same manner as an index-guided fibre guides light inside it. Only ultrafast lasers are capable of producing this effect in transparent materials. With longer pulse lasers (nanosec- onds), the sample damages before the intensity reaches the threshold at which guides are formed. The picture below shows the output of a waveguide made in glass on a far-field screen using a HeNe laser.
2.2. Motivation for synchrotron radiation 9 Another way to fabricate planar waveguides is to form a thin transparent film with increased refractive index on some substrate, or possibly embedded between two substrate layers. For example, one can embed a thin neodymium-doped YAG layer in an undoped YAG substrate with slightly lower refractive index.
Planar waveguides are used for optical amplifiers with high gain (compared to that of bulk amplifiers) and relatively high power. There are also planar waveguide lasers.
Some of these devices can be pumped with a proximity-coupled laser diode and do not require any pump optics.
2.2 Motivation for synchrotron radiation
By using photons of the X-ray regime of a synchrotron light source instead of UV laser exposure, one can overcome some of the restrictions due to inherent character- istics of X-ray radiation. Specifically:
Shorter wavelength:
The higher energy of the photons allows fibre cores to be penetrated without the need to strip the fibre beforehand because of the lower absorption in the coating.
Mask layout:
A proximity system can be used where a mask is inserted directly into the beam of light and blocks it in the desired regions. Hence it is feasible to design the affected area in a great variety of shapes and layouts by adjusting the design of the mask. This method also allows to process more than one fibre simultaneously.
Pre-treatment:
The changes of refractive index induced by synchrotron radiation are known to be persistent in untreated fibres.
Broader index change:
Since synchrotron radiation is not focused into the fibre core but passes it through the mask, the index of refraction modification does not only occur in the core but also in the cladding. This leads to sharper reflection/transmission spectra.
2.3 Previous research on synchrotron light
By mid last century groups of researchers were already showing interest in the various effects synchrotron radiation has on glass. In particular, in 1953 groups around
10 2. Introduction Primak reported on the effects of exposing quartz and silica to the radiation inside a nuclear reactor. They noted an increase in density and refractive index in vitreous silica [36]. Some years later, the same research group extended their studies to consider the effects of different radiation types, including neutrons, electrons, gamma and X-rays, on vitreous silica aiming to explain the mechanisms which lead to the occurrence of changes in the materials which could be observed. The Primak research was unable to determine any energy threshold for the observed compaction [37, 39];
even soft X-rays seemed to lead to the effect as efficiently and thus they concluded that the effect was caused by ionisation.
For most radiation types Primak’s group found that the compaction followed a 2/3rds power of dose and they interpreted it as a hardening effect [38].
In recent years it has been shown that the compaction induced by UV two-photon processes follows an almost identical dependence [41]. Hence it seems likely that the densification mechanism that occurs due to exposure to X-rays does not differ very much, if at all, to that reported in the UV. That would also explain why Primak et al. were not able to find a threshold energy since the energy from the highest occupied bonding orbital in SiO2 to the first unoccupied nonbonding orbital is of the order of 10 eV which is much lower than the energies that were investigated in those early experiments.
2.3.1 Synchrotron radiation applied to grating and wave- guide fabrication
Interestingly, no groups have reported on the fabrication of fibre Bragg gratings or waveguides in silica or germanosilica by means of synchrotron radiation. In related fields of research, however, synchrotron radiation has successfully been used.
In 2002, a change in refractive index of 0.04, an order of a magnitude larger than ever reported in germanosilica exposed to UV, was attained via compaction after irradiation with synchrotron radiation with a peak energy of 100 eV by Akazawa et al. [10]. Two years later, researchers around Kameyama [26] managed to increase the refractive index in glass by 4×10−3 by irradiating it with x-rays (peak energy
∼2.5 keV). A high O2 content glass was exposed to an X-ray intensity of 6.6 W/cm2 and experienced a refractive index increase of 7×10−3 (this value was reduced to 4×10−3 after thermal poling of the sample).
Directly writing of a grating in an embedded optical waveguide using synchrotron radiation was demonstrated by Kobayashi et al. [27]. The reported index change as large as 10−2 lead to a surprisingly low grating reflectivity of less than the 60 % expected for a index modulation of 10−4. The observed index changes were found to be due to a compaction process and the dose-compaction dependence seems to
2.3. Previous research on synchrotron light 11 follow the 2/3rds power relation reported by Primak in silica, which would support this hypothesis.
Although X-rays seem to be a promising alternative for modifying the refractive index in glass, researchers are trying to avoid the existing problems by investigating several other approaches such as two-photon processes and deep UV laser irradiation.
Chapter 3
Theory
3.1 Photosensitivity
Refractive index changes, induced by inhomogeneous irradiation of a material, have been known as an independent phenomenon since 1969, often referred to as the pho- torefractive effect. Its occurrence has been demonstrated not only in germanosilica glasses, but also in a wide range of material classes such as organic and inorganic crystals and liquid crystals. Photosensitivity refers to the amount of a change in the refractive index induced by exposure to electromagnetic radiation. Initially photo- sensitivity had been seen to be a phenomenon only associated with optical fibres, that contain a large amount of germanium in the core. Subsequent research however has shown photoexcitation in many different fibres, many of which contain several other dopants besides germanium and some of which do not contain any germanium at all. Nonetheless, due to their strong photosensitive response, germanosilicate fibres remain among the most important materials for the fabrication of devices utilising photosensitivity.
In an optical fibre, photosensitivity was first observed after exposing the fibre to laser light at 488 nm [23] and subsequently associated with a two-photon process.
3.1.1 Enhancing the photosensitivity in germanosilica
Untreated standard single-mode telecommunication fibres typically exhibit changes to the index of refraction as a result of irradiating with high energy UV light of approximately 3×10−5. In general increasing the amount of dopants will lead to a increased photoinduced index changes as large as∼5×10−4, which is still not large enough for many Bragg grating or waveguide applications. An order of magnitude
14 3. Theory larger index change (i.e. 10−3) would typically be required in order to achieve a 99.9 % reflectivity grating over a length of 2.5 mm, for instance [44].
Since it is often desirable, however, to write photoinduced devices into standard optical fibres, various sensitisation techniques have been developed to increase the photoinduced index modulation. These include the Hydrogenation, Flame Brushing and Hypersensitisation discussed below.
3.1.1.1 Hydrogenation (Hydrogen Loading)
Hydrogenisation increases the number of oxygen deficiency centres in, and thereby the photosensitivity of, germanosilica fibres by diffusing hydrogen molecules into the fibre core at high pressure and temperature. If fibres are hydrogenated prior to exposure, an order of magnitude increase in reflectivity of the grating can be achieved [31].
During the progress of hydrogen loading the fibres are soaked in a hydrogen gas within a temperature range of 20-75 and pressures from ∼20 atm to more than 750atm. This process allows the fabrication of gratings in fibres with a low germa- nium concentration, which is typical for standard communication fibres, and hence a low intrinsic photosensitivity. And since the increased photosensitivity due to hy- drogenisation is not a persistent effect, the permanent refractive index changes only occur in areas that are irradiated. In other parts of the fibre the unreacted hydrogen simply diffuses out over time. The diffusion of the hydrogen can cause a drift in the FBG wavelength. However this can be avoided by annealing the fibre.
0 200 400 600 800 1000 1200 1400
0,0 5,0x10-4 1,0x10-3 1,5x10-3 2,0x10-3 2,5x10-3 3,0x10-3 3,5x10-3
∆n
Cumulative Fluence (J/cm2)
Ge High Ge B/Ge Hydro Ge Hydro High Ge Hydro B/Ge
Figure 3.1: Evolution of photo-induced refractive index changes for fibres with different germanium concentrations in the core (code names are explained in the text). Reproduced from Ref. [28].
3.1. Photosensitivity 15 Figure 3.1 on the preceding page illustrates the effect of hydrogen loading on fibres with a different Ge concentration in their cores (Ge: Low loss germanosilica fibre, Ge concentration 5 mol %; High Ge: High-index germanosilica fibre, 20 mol %; B/Ge:
Boron co-doped fibre,∼20 mol %).
As can be seen from the graph, the photoinduced refractive index change for the low-loss germanosilica fibre (Ge) is, as expected, the lowest among the fibres ex- amined. The high-index germanosilica (High Ge) and the boron co-doped (B/Ge) fibres exhibit similar behavior, with the boron co-doped fibre being slightly (10 %) more photosensitive. Both fibres reach saturation above 200 J/cm2, with a refractive index change of 3.8×10−4 for the High Ge and 4.5×10−4 for the B/Ge, increasing very slowly thereafter. On the Ge fibre however, hydrogenation only has a minimal effect, increasing its photosensitivity by a factor of 2.5.
Using deuterium instead of hydrogen pushes the infra-red absorption peaks out to longer wavelengths [21] and thus avoids the broad absorption peaks in the IR at 1.39 µm and 1.41 µm resulting from the hydrogen loading.
3.1.1.2 Flame brushing
This is a simple and effective technique for enhancing the photosensitivity of ger- manosilicate fibres. The fibre is brushed repeatedly, in the region to be photosen- sitised, by a flame that is fueled with hydrogen and a small amount of oxygen at a temperature of ∼1700 . Similarly to the hydrogenation process, flame brushing causes hydrogen to diffuse at these high temperatures very quickly into the fibre core, where it reacts with germanosilicate, creating a strong absorption band at 240 nm and rendering the core highly photosensitive. As opposed to hydrogen loading, the increase in photosensitivity through flame brushing is permanent. Localisation can be achieved by using a relatively small flame. However, one major drawback of this approach is a loss of long-term stability of the fibre as the high temperature weakens the material.
3.1.1.3 Hypersensitisation
By initial sensitisation of a material through optical or thermal exposure, the pho- tosensitive response of the material for subsequent processing can be enhanced. For instance, the intrinsic 224 nm photosensitivity of boron-codoped germanosilicate op- tical fibres can be enhanced by pulsed UV radiation. Hypersensitisation has been reported with pre-exposures at 193 nm, 248 nm and 355 nm [17].
3.1.2 Models for photosensitivity
Validating one single model which describes photosensitivity in germanosilicate fibres leads to difficulties as there is usually more than one mechanism involved. Moreover, the fibre (type, doping, temperature), radiation (wavelength, power, fluence) or
16 3. Theory
0 5 10 15 20 25
0 2 4 6 8 10 12 14 16 18
Refractive index increase (10-4 )
Flame-brush time (min)
Figure 3.2: Plot of the maximum (saturated) photoinduced index change observed in Corning SMF-28 fibre as a function of processing time under the flame brush.
Reproduced from Ref. [15].
preprocessing (as described above) can all play a role in causing the changes that occur in any given sample.
The current consensus explains photosensitivity as being initiated through the forma- tion of colour-centres [24] and compaction of the irradiated glass [22]. However, both of colour-centres and compaction are themselves not fully understood yet. Moreover, whether colour-centres or compaction serves as the dominant effect is determined by reference to the range of experimental factors considered above, which obviously vary from case to case.
The theories underpinning these two models are described below. For a wider overview over different photosensitivity models and these two in particular refer to [35].
3.1.2.1 The Colour-centre Model
A colour-centre or F-centre (derived from the German wordFarbzentrum for colour- centre) refers to where there is a missing anion causing a vacancy that is filled with one or several electrons. The resulting oxygen deficiency in the silicate glass matrix is called Ge-Si or Ge-Ge “wrong bond”. Color-centres cause the atom to absorb photons of a different frequency (colour) after irradiation. Such defects are important where optical fibres are concerned because their absorption bands cause deleterious transmission losses. Colour-centres are also responsible for non-linear optical fibre transmission, where the transmission changes over time and with light intensity.
Consequently, a great deal of research has been directed towards minimising the
3.1. Photosensitivity 17 formation of colour-centre defects in glass. With their link to fibre Bragg gratings, however, they are playing a big role within fibre optics and the colour-centre model has received a great deal of attention.
According to the colour-centre model of photosensitivity, the colour-centre defects that are formed during the fabrication process of germanosilicate fibres are responsi- ble for their intrinsic photosensitivity by absorbing UV radiation in the wavelength range between 240-250 nm (∼5 eV), which leads to bleaching of the colour-centres and a growth of absorption features at shorter wavelengths (see Figure 3.3).
200 220 240 260 280 300 320 340 360
0 500 1000 1500 2000 2500
Loss (dB/cm)
Wavelength (nm) After UV irradiation
Before UV irradiation
Figure 3.3: Transmission loss spectrum of a fibre preform before and after irradi- ation with 244nm UV light. Reproduced from Ref. [34].
The colour-centre model was first proposed by Hand and Russell in [24]. They dis- covered that photoinduced changes in the material properties of the glass introduce new localised electronic excitations and transitions of defects. Due to their strong op- tical absorption, it is contended that it is precisely these colour-centre defects which give rise to the change in the refractive index associated with photosensitivity. The bleachable wrong bond defects, that initially absorb the light, are transformed into defects that are more polarisable by virtue their electronic transitions take place at longer wavelengths or that they have stronger transitions.
Any change in the refractive index is associated with the photoinduced change in absorption trough the Kramers-Kronig relation, given as
∆nef f(λ) = 1 2π2P
Z ∞
0
∆αef f(λ)
1−(λ/λ0)2 dλ (3.1) whereP is the principal part of the integral, λis the wavelength, and αef f(λ) is the effective change in the absorption coefficient of the defect which is given by
∆αef f(λ) = (1/L)
Z L
0 ∆α(λ, z) dz (3.2)
18 3. Theory whereLis the sample thickness. In this manner, it is factored in that the bleaching beam is strongly attenuated as it passes through the sample, and therefore bleaching does not occur uniformly with increasing depth. A Gaussian distribution provides an appropriate model for ∆αef f(λ) and the Kramers-Kronig relation in Equation 3.1 on the previous page may be used to calculate the change in the refractive index induced by bleaching of the absorption bands.
Many studies have indeed reported index changes that are consistent with estima- tions made by means of the colour-centre model, thus supporting its application as a model suitable for assessing photosensitivity [18].
Also supporting the colour-centre theory are findings that the bleaching of the 240 nm band can be reversed by heating the fibre. It is known that thermally acti- vated processes serve to anneal colour-centres by exciting the trapped electrons out of the anion vacancy sites. A few groups report to have written and then completely erased gratings by annealing at temperatures above 800 [13, 18].
3.1.2.2 The Compaction Model
Albert et al. have reported in [11] a growth rate of fibre Bragg gratings that was linearly proportional to the 193 nm laser pulse energy density in fibres highly doped with germanium, but proportional to the square of the pulse energy density for standard telecommunication fibres with low germanium concentration. Furthermore, the achieved index changes in the latter case were an order of magnitude greater than previously achieved in fibres without sensitisation treatment. Further experiments with fibres treated to entirely remove germanium oxygen deficiency centres ruled out both E’ colour-centres and oxygen deficiency centres as a cause for the index change.
A model that explains the change in refractive index considering the mentioned observations is the model of densification or compaction of the material when being irradiated with high energy light.
According to the compaction model, exposure of SiO2 glass to radiation can cause a local increase of density i.e. volume compaction. Since the refractive index is known to be linked to the material density (see Figure 3.4 on the facing page), a local increase in density will, in general, lead to a local increase in refractive index.
This increase can be estimated by means of the Lorentz-Lorentz relation
∆n
n = ∆ρ
ρ (1 + Ω)(n2−1) (n2+ 2)
6n2 . (3.3)
For densified silica glasses the relationship between refractive index and density has been found to be linear as shown in Figure 3.4 on the next page
From [42] we obtain the functional relation for n(ρ) to be
n(ρ) = 1.037 + 0.195·ρ. (3.4)
3.1. Photosensitivity 19
2,15 2,20 2,25 2,30 2,35 2,40 2,45 2,50 2,55 2,60
1,46 1,48 1,50 1,52 1,54
Refractive Index
Density (g/cm3)
Figure 3.4: The relationship between density and refractive index in silica glass.
Reproduced from Ref. [42].
Primak established a mathematical description of the compaction κ depending on the deposited dose D in the material [40]. Compaction is hereby defined as the negative change in volume
−κ= V0−V
V0 = ∆V
V0 (3.5)
where ∆V is the difference between the original volume V0 and the present volume V after the process of densification. According to Primak’s studies, the compaction follows a 2/3rds power relation of the deposited dose:
κ=C·D2/3 (3.6)
where C denotes a constant factor and D is the dose profile. The compaction occurring in fibre Bragg gratings in germanium-boron codoped fibres and hydro- genised fibres was observed by means of AFM measurements in research completed in 2002 [43].
The exact mechanism by which compaction occurs is not fully understood, however it can be taken for granted that radiation-induced volume compaction is achieved by a two-photon effect and is a result of reorganisation of existing bond structures rather than bond rupturing or the formation of defect centres. Research by different groups [16, 33] indicate that a relaxation process may provide a means of crossing an energy barrier and thus allowing the system to be in a lower free-energy state.
The processes leading to compaction appear to be triggered by ionisation events that occur when samples are exposed to radiation with an energy high enough to bridge the SiO2 band gap of ∼8 eV. This provides compelling arguments for writing re- fractive index structures at shorter wavelengths via a compaction mechanism rather
20 3. Theory than the common method using the 240 nm (∼5 eV) absorption band. Whereas this requires pretreatments of the fibres to increase their photosensitivity as well as post-annealing in order to prevent Bragg wavelength shifts, index changes can be induced by compaction in untreated fibres and tend to be more stable.
3.2 Synchrotron radiation
Synchrotron radiation is associated with a change in velocity (i.e. acceleration) of electrons as they spiral around a magnetic field. Accidentally discovered in 1947, at the General Electric Company’s synchrotron accelerator, the electromagnetic radia- tion released from a synchrotron light source is therefore referred to as synchrotron radiation or synchrotron light.
Accelerated electrons radiate characteristic photons of the frequency ν = eB
mc (3.7)
where B is the magnetic field, e and m are the electric charge and the mass of the electron respectively andc is the speed of light.
bending magnet with
magnetic field electron beam
tangentially emitted synchrotron light
Figure 3.5: An electron is accelerated in a magnetic field and emits synchrotron radiation tangentially to its velocity.
Synchrotron radiation has several distinct properties that make it an important instrument for various applications:
Extremely high brightness
Synchrotron light is generated when a thin electron beam is diverted by a mag- netic field. Synchrotron light at X-ray wavelengths can be a million times as bright as X-rays produced from a conventional source (see Figure 3.6(a)). This characteristic permits researchers to explore the internal structure of matter with great clarity.
Continuous spectrum
A synchrotron light source provides a method of generating electromagnetic
3.3. Ellipsometry 21 radiation that has a continuous, adjustable and broad spectrum, shown in Fig- ure 3.6(b). With a monochromator on the beamline, a synchrotron light source can separate wavelengths concurrently over a wide range including infrared, visible and UV light, and X-rays.
10-7 10-5 10-3 10-1 101 103 105 107
10-4 10-3 10-2 10-1 100 101 102
106 108 1010 1012 1014 1016 1018 1020
Brightness (Photons/s/mrad2/mm2/0.1%BW)
Photon Energy (keV) Synchrotron light produced by NSRRC undulators
Synchrotron light produced by NSRRC wigglers
Solar radiation
Synchrotron light produced by NSRRC bending magnets
Characteristic X-ray produced by X-ray generator
White light produced by XX-ray generator
Photon Energy (eV)
GAMMA RAYS HARD X-RAYS SOFT X-RAYS
INFRARED
MICROWAVES ULTRAVIOLET
RADIO WAVES VISIBLE
Range covered by the NSRRC
(a) (b)
Figure 3.6: Brightness and radiation spectrum of the NSRRC.
Highly collimated Beam
Because synchrotron light is emitted tangentially to the orbital direction within a tiny angle, the photon beam remains completely in focus at an experimental station metres away.
Small beam cross section
The cross section of a synchrotron beam can be less than 0.02 mm2, syn- chrotron light thus offers high angular resolution and can be used to study fine samples.
Pulsed time structure
The synchrotron electrons exist in the form of a series of pulses, thus the synchrotron light is emitted in pulses. This feature is essential for studying the dynamics and time dependence of various phenomena.
Polarisation
Synchrotron light can have linear, elliptical or circular polarisation. This prop- erty is extremely useful for research into the symmetry of electron energy levels and the geometric structure of surfaces.
3.3 Ellipsometry
Ellipsometry is a very sensitive optical method that is based on the fact that the polarisation state of light may change when the light beam is reflected from a surface.
22 3. Theory A thin film or a stack of films deposited on a substrate will influence the polarisation as it shifts the phase and changes the amplitude of the two polarisation states. It is therefore possible to deduce information about the film properties such as film thickness and index of refraction.
3.3.1 Polarisation
Generally monochromatic light is elliptically polarised. If the phase of the x and y oscillations is equal, the resulting ellipse degenerates into a straight line. By contrast a phase difference of ±90◦ causes the ellipse to become a circle. Thus, linear and circular polarisation are specialised cases of the general elliptical state.
If a light beam illuminates a surface under oblique incidence, the plane of incidence (Figure 3.7) is defined by the wave vectork pointing in the direction of travel of the light and the surface normal n. It is now common to define the direction of x and y in such a way that x is parallel to the plane of incidence and y is perpendicular.
These directions are referred to as p (for parallel) and s (for perpendicular) and replace thex, y notation. Thus, the electric fieldE of the wave is resolved into its p and s components.
p-plane
s-plane
Ein
Incident beam, linearly polarised
Sample
Reflected beam, elliptically polarised
Φ0
Eout Ψ
∆
Figure 3.7: The plane of incidence and polarisation states of incident and reflected beam.
The light is reflected by the surface of the sample which may constitute a complex optical system with various layers of different optical properties. Multiple reflection at the interfaces of the layers superimpose to finally form the reflected light wave with an altered state of polarisation. In particular, the p and s components will undergo different overall phase shifts and also exhibit altered reflective properties.
Thus, the shape and the size of the ellipse of polarisation are changed. This change again is a measure of the properties of the optical system. The incident and reflected E vectors are connected by the reflection matrix R of the sample:
Ein=
Ep,in Es,in
=
Ep,in0 eiδp,in Es,in0 eiδs,in
Eout respectively (3.8)
Ep,out Es,out
=
Rpp Rsp Rps Rss
Ep,in Es,in
. (3.9)
3.3. Ellipsometry 23 The concept of ellipsometry is to measure the change of the polarisation state of the light wave to obtain information about the sample (the matrix R).
3.3.2 Phase-modulated ellipsometry
Introduced in 1968 by Jasperson and Schnatterley, phase-modulates ellipsometry is an approach to the analysis of the state of polarisation, that uses a strained piece of amorphous silica to modulate the state of polarisation of the light. The underlying principle of phase-modulation is described in [20,25]. The silica becomes birefringent when strained, with the amount of birefringence (the phase retardation of a light beam passing through the optical element) proportional to the strain. The strain is applied by piezoelectric transducers at the resonance frequency of 50 kHz. The resulting detector signal has a large unmodulated component, with two superimposed modulated signals at ωM=50 kHz and 2ωM=100 kHz (and higher harmonics). The so-called ellipsometric parameters ∆, Ψ (see below) can be directly deduced from these modulated signals, which can be easily separated by phase sensitive detection through lock-in amplifiers. The technique allows a fast response time and has a superior signal-to-noise ratio due to the use of lock-in detection and a high signal throughput.
A simplified schematic of such an instrument is presented in Figure 3.8.
Sample Polariser
Photoelastic modulator Analyser
Xe light source
Monochromator Filters
Detector
Data acquisition and computing Shutter
∆, Ψ
Figure 3.8: Simplified schematic of a multichannel ellipsometer based on the phase modulation principle. The source consists of a collimated Xe lamp, and the detection system includes a monochromator and a linear array of silicon photodiodes.
The phase-modulation ellipsometer is a photometric instrument since its operation typically involves directing a quasi-monochromatic light beam through the polarisa- tion generation and detection arms of the instrument and measuring the output of the detector as a function of time over several periods of the modulator [14]. The major system components include:
Xe light source with collimating optics
24 3. Theory
A fixed polariser
The reflecting sample
A photoelastic modulator
A fixed analyser
A monochromator
A linear photodiode array.
3.3.3 Performing ellipsometry measurements
The process of data acquisition with ellipsometry is illustrated in Figure 3.9. It com- prises the four basic step of measuring, modeling, fitting the model and extracting the optical properties from the model.
Measurement Model Fit Results
Exp. Data
0 z-1 z
Compare model to fit and get fit parameters
n, k, thickness, ...
improve the model z layers, n, k
Figure 3.9: The steps of the process of data acquisition by means of ellipsometry.
Measuring the matrix R
For isotropic materials R is diagonal (Rsp, Rps = 0) and the ellipsometric angles Ψ and ∆ can be defined, describing the ratio of the complex reflection coefficientsRpp
and Rss, which are the values that are actually measured by the ellipsometer:
Rpp
Rss = |Rpp|
|Rss|ei(δpp−δss) (3.10)
∆ = δpp−δss (3.11)
tan Ψ = |Rpp|
|Rss|ei∆. (3.12)
Equation 3.12 is the fundamental relation of ellipsometry.Here, Ψ denotes the relative phase shift of the p and s component upon reflection, while ∆ is an angle whose tangent gives the ratio of amplitude change for thepandscomponents. For ∆ = 90◦ the sensitivity of the ellipsometer reaches its maximum. This angle is given if the incident light hits the sample under the Brewster angle of the material.
The result of the measurement is a set of anglesP,C andA for the polariser, com- pensator, and analyser respectively. From these angles the elements of the reflection matrix can be derived.
3.3. Ellipsometry 25
Modeling the optical properties
However, the matrixR itself is not significant in terms of the physics of the problem since it does not give any values for physical quantities of the sample under exam- ination. These quantities can, in general, not be calculated from the ellipsometric angles, because the formulas that describe R as a function of these parameters are complicated and can not be inverted in cases other than particular simple ones.
Therefore, one has to develop an optical model (i.e. a formula for ∆ and Ψ as a function of the desired physical parameters) and fit the output of the model until it equals the measured values of ∆ and Ψ.
From a single measurement two real quantities can be obtained. Thus it is in prin- ciple possible to measure a combination of two real numbers. However, often it is needed to measure more than two parameters. To accomplish this task the number of independently measurable quantities has to be increased. In spectroscopic ellip- sometry, this is done by measuring at different wavelengths, where each wavelength introduces a new unknown refractive index (due to dispersion) but provides two new values for ∆ and Ψ.
Because ellipsometry measures the ratio of two values, it can be highly accurate and very reproducible. The ratio is a complex number, thus in the angle ∆ it contains
“phase” information which makes the measurement very sensitive.
Fitting the data to the model
Having established a model that describes the system, the experimental data has to be fitted to this model. It is important that the starting values for the fit are reasonably chosen. If the starting values of the unknowns are too far off, the regres- sion algorithm will get lost. A mean squared Error (MSE) is used to quantify the difference between experimental and calculated model data. A smaller MSE implies a better fit.
Data evaluation
Once the fit is complete, the resulting fit parameters must be evaluated for sensitivity and possible correlation. The MSE might be reduced further by refining the model and increasing its complexity. It also needs to be made sure that the fit parameters are actually physical.
If the model matches the data accurately and all parameters are within a reasonable range, it is very likely that the fitted model actually reflects the given physical system.
Chapter 4
The synchrotron experiments
4.1 The National Synchrotron Research Center
Located in Hsinchu Science Park, Taiwan, the synchrotron accelerator of the Na- tional Synchrotron Radiation Research Center (NSRRC) was opened in October 1993. It was the first synchrotron light source of the third generation built in Asia.
By the end of 2004, twenty-seven beamlines had been completed at the NSRRC, each providing a varied flux, brightness and spectral range. In total, the NSRRC possesses more than fifty experimental stations located at the beamlines, covering experimental technology of all types such as chemical dynamics, a photoemission electron microscope, an inelastic X-ray scattering end station and a X-ray lithogra- phy beamline.
Figure 4.1 represents a schematic of the synchrotron in Taiwan:
Insertion device (wiggler/undulator)
Sextupol magnet
Bending magnet Quadrupol magnet
RF cavity
Storage ring
Transport line Beamline
LINAC
Booster ring Experimental
station
Figure 4.1: Layout of the NSRRC. Reproduced from Ref. [9].
High energy electrons produced in a booster ring (1) enter the storage ring (3) via a transport line (2); in the storage ring, the electrons produce synchrotron light after
28 4. The synchrotron experiments being deflected by bending magnets or insertion devices, and the emitted radiation is collected and channeled through beamlines (4) to experimental units (5) at which the experiments are eventually performed.
1. Booster ring
A linear accelerator (LINAC) accelerates an electron beam produced with an electron gun to an energy of 50 MeV. The electron beam then enters a booster ring of 72 m diameter and increases its energy to 1.5 GeV, reaching 99.999995 % of the speed of light.
2. Transport line
From the injector, the electron beam passes the transport line into the storage ring. The transport line has a length 70 m.
3. Storage ring
The electron beam then enters the hexagonally shaped storage ring, which has a diameter 120 m. Bending magnets located around the ring steer the motion of the electrons along an orbital path, the electron beam thus continuously emits synchrotron light tangentially to the direction of their movement. To ensure the stability of the electron beam, an absolute vacuum condition of less than 1×10−10 Torr is maintained within the storage ring. The electrical current of the beam is 200 mA, while approximately 50 billion electrons are orbiting inside.
4. Beamlines
The beamlines serve as links between the synchrotron light source and the experimental stations. In principle, to direct the synchrotron light to an ex- perimental unit, a port might be opened at each location at which electrons are deflected, or directly downstream from each insertion device.
5. Experimental unit
On arriving at the experimental unit, the synchrotron light is applied to an experimental sample. Depending on the type of measurement a great variety of experiments can be set up.
The insertion devices used in the NSRRC comprise magnets in rows with alternating polarity, which deflect the passing electron beam multiple times. If the intensity of the magnetic fields is increased, the frequency of emitted light can increase to the level of soft X-rays, or even hard X-rays. Such a device that increases the energy of the light is known as a wiggler. If the spatial frequency of the alternating magnetic fields is shrunk, the magnitude of the electron motion is tapered. Thereby the synchrotron light gains constructive interference at a particular wavelength, at which the brightness of light is greatly enhanced. A device increasing the brightness of the light is known as an undulator.
4.1. The National Synchrotron Research Center 29
4.1.1 The beamline
The beamline used for the synchrotron experiments was the micro-machining/LIGA beamlineBL18B. The acronym LIGA is derived from the German words Lithogra- phie, Galvanoformung and Abformung. LIGA signifies the use of photolithography to produce precision moulds, which are then usable to produce microstructures in large quantities. LIGA is especially suitable for mass production of highly precise microstructures with large aspect ratios. This beamline provides photon beams with energies above 500 eV from the bending magnet and is designed for LIGA related studies. It is applicable for high-speed exposure processes with an exposure area big enough for a six-inch wafer.
Re-focussing mirror
Exit slit and gate valve
To experimental station
Monochromator
Entrance slit and gate valve
Horizontal focussing mirror
From light source Vertical focussing mirror
Figure 4.2: Schematic of the micro-machining/LIGA beamline.
The layout of the LIGA beamline is illustrated in Figure 4.21. Focusing mirrors focus the photon beam horizontally and vertically. The entrance and exit slits allow for control of intensity and resolution of the focused beam. A monochromator (grating or crystal) can be used to select light of a particular wavelength. The re-focusing mirror focuses the beam of light on a sample in the experimental station.
Gate valves and metal bellows split the beamline into three major sections. Each is equipped with its own independent pumping system. The first section splits the light and guides a beam with a beam width of 10 mrad to the second section. A photon shutter and diamond window are located at the end of the second section.
The third section is a low-conductance delay line. A sensor detects any possible vacuum failure in the exposure chamber and protects the upstream UHV condition.
During maintenance of the wiggler beamline, this delay line can be removed.
4.1.2 The experimental unit
The experimental unit is manufactured byJENOPTIK Technologie GmbH.
4.1.2.1 The filter chamber
The filter chamber is located directly after the beam pipe. It operates under high vacuum and serves to accommodate aluminium filter foils which can be inserted into the beam. In this manner the X-ray radiation spectrum can be adjusted or limited.
4.1.2.2 The work chamber
The work chamber is sealed off from the filter chamber and represents the next section of the experimental unit. It is separated from the radiation-source vacuum
